Women are more susceptible to several aspects of drug addiction than men, including relapse following stressful events. Addictive processes critically involve hippocampal circuitry that supports spatial and episodic memory acquisition processes. In contrast to the impaired cognition observed in males after chronic stress, females display enhanced spatial memory following chronic stress suggesting ovarian hormone involvement. Within the dorsal hippocampus, it is well established that ovarian steroids, in particular estrogens, can modulate CA1 pyramidal cell activity and long-term potentiation (LTP), the cellular model of learning. During the last grant period we found that ovarian hormones also regulate enkephalins and the mu- and delta-opioid receptors (MORs and DORs, respectively) in a manner that could promote learning processes under certain conditions. Specifically, at proestrus (high estrogen): 1) enkephalin levels are elevated in mossy fibers (MFs), which synapse on the dendrites of CA3 pyramidal cells; 2) increased MORs are present on the plasma membrane of parvalbumin GABAergic interneurons, which inhibit CA3 pyramidal cells; 3) fewer DORs are present on the plasma membrane of pyramidal cells; and 4) MF stimulation may induce an opioid-dependent potentiation of CA3 field excitatory post- synaptic potentials. Chronic stress, which can trigger the release of opioid peptides, has maladaptive morphological responses in males that are not seen in females, and may also elevate MOR expression in parvalbumin interneurons in females. This renewal application proposes to test the central hypothesis that chronic stress leads to adaptive changes in the opioid system of females to promote CA3 LTP and other plastic processes that support drug-related learning. Approaches using a combination of light and electron microscopic immunocytochemistry, RT-PCR, in situ hybridization, in vitro slice electrophysiology and immobilization stress will test this hypothesis. Aim 1 will determine if chronic stress alters: 1) the levels and/or subcellular distribution of enkephalins within MFs and in lateral perforant path (LPP) afferents to CA3 and 2) opioid-mediated LTP and/or other forms of opioid-mediated plasticity at MF/LPP- CA3 synapses in a manner promoting learning processes in females. Aim 2 will determine if following chronic stress MORs and DORs have altered: 1) expression and cellular distributions; 2) trafficking within select cell types; and/or 3) phosphorylated levels or distributions in the CA3 region in a manner promoting excitation and learning processes in females.